What is the system being used for positioning in space?

As everything is moving in space, you need to consider some celestial body as "stationary" and describe others' position relative to the reference point.

To learn more and in depth,
you may borrow To rise from earth : an easy-to-understand guide to spaceflight by Wayne Lee.
There is some detailed but simple explanation about the coordinate system used in space

P.S. If you like astronomy, this book is quite good.

2006-10-24 01:02:34 補充：
You're welcomed ^^
You'll love the book